There is considerable interest in using rare earth doped fiber amplifiers to amplify weak optical signals for both local and trunk optical communications networks. The rare earth doped optical amplifying fibers are found to have low cost, exhibit low-noise, provide relatively large bandwidth which is not polarization dependent, display substantially reduced crosstalk problems, and present low insertion losses at the relevant operating wavelengths which are used in optical communications. Contemplated rare earth doped optical fiber amplifiers can be coupled end-to-end to a transmission fiber and transversely coupled, through a directional coupler, to a laser diode pump. The directional coupler is designed to have a high coupling ratio at the pump wavelength and a low coupling ratio at the signal wavelength so that maximum pump energy is coupled to the amplifier with minimal signal loss. When the amplifying medium is excited with the pump laser, signal light traversing the amplifier experiences a gain. The pump energy may be made to propagate either co-directionally or contra-directionally relative to the signal energy, depending upon whether any remaining unconverted pump light can be more conveniently filtered at the transmitter or the receiver.
A complicating factor in the design of rare earth doped optical amplifiers involves the difference between the various parameters necessary to optimize the performance of the amplifier and those necessary to optimize the performance of the associated transmission fibers. These differences, which arise from the different functions performed by the optical amplifier and the transmission fiber, result in significant signal loss as the signal is transmitted from the transmission fiber to the amplifying fiber, and therefore place a premium on the efficiency of the amplifying fiber which restores the signal to its previous levels. In the transmission fiber, waveguide dispersion must be minimized in order to maximize bandwidth and minimize loss, thereby maximizing the spacing between repeaters. However, in the amplifying fiber, as opposed to the transmission fiber, the major concern involves high gain, high saturation power, and low noise. Exemplary signal losses which can occur because of the different optimal parameters for the transmission and amplifying fibers are splicing losses due to mode mismatch because the signal mode size may be significantly different for the two fibers.
To date, erbium fiber amplifiers appear to have the greatest potential for the high amplification necessary to overcome the signal losses due not only to normal signal processing but also to the mismatch which can occur between a tansmission fiber and an amplification fiber. Erbium doped fiber amplifiers operate at .lambda.=1.53 .mu.m which is of particular interest for optical communication systems because this lasing transition falls into the low-loss window of fiber optic communications. In addition, at this wavelength region, the amplifiers exhibit low insertion loss, broad gain bandwidth (approximately 30 nm) and gain which is not polarization sensitive. A solution to the problem of establishing and fixing the various parameters for providing an erbium-doped fiber amplifier which operates in a most efficient mode is required.